The invention relates generally to plasma-arc torch systems and power supplies. In particular, the invention relates to systems, circuits, and methods for controlling contact starting and operating plasma-arc torches, including controlling a gas control solenoid, a power supply, and contact start elements.
Plasma-arc torches, also known as electric arc torches, are commonly used for cutting, welding, and spray bonding workpieces. Such torches typically operate by directing a plasma consisting of ionized gas particles toward a workpiece. An example of a conventional gas plasma-arc torch is disclosed in U.S. Pat. No. 3,813,510, the entire disclosure of which is incorporated herein by reference.
In general, a pressurized gas to be ionized is supplied to the front end of the torch (also referred to as the torch head) and flows past an electrode before exiting through an orifice in a torch tip. The electrode has a relatively negative potential and operates as a cathode. The torch tip, which is adjacent the electrode at the front end of the torch, constitutes a relatively positive potential anode. When a sufficiently high magnitude voltage is applied to the electrode, an arc is established across the gap between the electrode and the torch tip, thereby heating the gas and causing it to ionize. The ionized gas in the gap is blown out of the torch and appears as a flame extending externally from the tip. The arc so established is commonly referred to as a pilot arc. A typical pilot arc circuit may provide, for example, 5-50 amps, at 100-200 volts across the electrode to tip gap.
Plasma-arc torches may be found in both xe2x80x9cnon-contact startxe2x80x9d and xe2x80x9ccontact startxe2x80x9d varieties. In non-contact start torches, the tip and electrode are normally maintained at a fixed physical separation in the torch head. Typically, a high voltage high frequency signal (HVHF) is applied to the electrode (relative to the tip) to establish a pilot arc between the electrode and the tip. This may be referred to as HF starting. HF starting generally requires additional circuitry that can cause undesirable electromagnetic interference (EMI) conditions. Regardless of how a pilot arc is established, when the torch head is moved toward the workpiece, the arc transfers to the workpiece-assuming a conductive (e.g., metal) workpiece that is connected to the positive return.
In a typical contact start torch, the tip and/or electrode make electrical contact with each other (e.g., along a longitudinal axis of the electrode). For example, a spring or other mechanical means may be used to bias the tip and/or electrode such that the tip and electrode are normally in electrical contact when gas is not flowing. When the operator squeezes the torch trigger (also referred to as a torch activation switch), a voltage is applied to the electrode and pressurized gas (the plasma gas and/or a secondary gas) flows. The gas causes the tip and electrode to overcome the bias and physically separate. As the tip and electrode separate, a pilot arc is established therebetween.
There are several ways, mechanically speaking, to create the electrical contact necessary to employ a contact starting process. For example, a fixed electrode and translatable tip configuration is possible. In such a configuration, a spring or other means biases the tip into contact with the electrode. When a gas control solenoid opens and supplies plasma and/or secondary gas, the gas flow overcomes the bias force and separates the tip from the electrode, thereby establishing a pilot arc. This configuration is typically referred to as a blow forward contact start torch. Another example involves a fixed tip and translatable electrode that is biased into electrical contact with the tip. In such a configuration, the flow of plasma and/or secondary gas overcomes the bias and separates the electrode from the tip to establish the pilot arc. This configuration is typically referred to as a blow back contact start torch. Both of these exemplary configurations may be referred to as blow apart torches because they employ gas pressure to separate the tip and electrode during the contact start process. Mechanical and/or electromechanical contact starting means are also possible.
Commonly owned U.S. patent application Ser. No. Ser. No. 09/724984, filed Nov. 28, 2000, the entire disclosure of which is incorporated herein by reference, describes contact start torch operations in the context of a circuit and method for ensuring that the parts of a contact start plasma-arc torch are properly in place before allowing the output voltage to ramp up to its final value. Commonly owned U.S. Pat. No. 5,961,855, the entire disclosure of which is incorporated herein by reference, describes a contact start torch in context of a low-voltage source for conducting a parts-in-place check.
In order to use a plasma-arc torch with a workpiece, a main or cutting arc must normally be established between the electrode and the workpiece. As the torch head or front end is brought toward the workpiece, the arc transfers between the electrode and the workpiece because the impedance of the workpiece to negative is typically lower than the impedance of the torch tip to negative. During this xe2x80x9ctransferred arcxe2x80x9d operation, the workpiece serves as the anode.
Once the arc transfer is sensed, it is generally preferred to cease current flow between the electrode and the tip. One method of terminating current flow between the electrode and the tip is to open circuit the pilot arc current path. This may be accomplished by sensing the presence of a current flowing in the workpiece and open circuiting a switch between the tip and ground (positive return). Commonly owned U.S. Pat. Nos. 5,170,030, and 5,530,220, the entire disclosures of which are incorporated herein by reference, describe an arc transfer process.
After arc transfer occurs, the output current is typically increased to a higher, cutting level. The power supply preferably is current controlled so that the cutting current is maintained at or near a constant current level. If the transferred arc is stretched beyond the capacity of the power supply it can extinguish. The arc may stretch, for example, when cutting a discontinuous workpiece (e.g., a metal grate), when cutting near the end of a workpiece, or when the torch is moved away from the workpiece. Once the arc has been extinguished, the torch starting process must typically be repeated. As can be appreciated, it is often desirable to restart the torch as quickly as possible. Commonly owned U.S. patent application Ser. No. 09/870,272, filed May 30, 2001, the entire disclosure of which is incorporated herein by reference, describes systems and methods for re-attaching the pilot arc before the transferred arc completely extinguishes, thereby reducing the likelihood of having to restart the torch.
In plasma arc torch systems employing HF starting, plasma and/or secondary gasses are usually turned on and allowed to run for a brief time before striking an arc. This allows the flow to reach a maximum level before the pilot arc is ignited. A gas pressure switch may be positioned between a gas control solenoid and the torch head to prevent pilot arc ignition until sufficient pressure is sensed, thereby ensuring the availability of plasma gas and that the solenoid properly opened. In prior art contact start torch systems, however, where gas pressure may be necessary to separate the tip and electrode, the gas control solenoid is normally opened at substantially the same time that a DC voltage is applied to the tip and electrode in the starting process. Accordingly, the pressure switch arrangement employed in HF starting systems does not provide an indication that the solenoid has operated properly (and allowed gas flow) before voltage is applied to the torch parts.
FIG. 1 illustrates a prior art contact start process. In particular, FIG. 1 illustrates the contact start process associated with a prior art blow apart torch. Blocks 102-112 reflect the contact start process up to the point at which the electrode and tip make contact with power applied. Thereafter, at block 114, a time delay accounts for the expected worst-case time for the electrode and tip to make contact in a blow apart torch. Blocks 116-126 generally reflect normal torch operations after a pilot arc is established during the contact start process.
As can be appreciated, the prior art time delay safety margin approach (block 114) is less than optimal and will slow down operation of a torch that is capable of restarting faster than the predefined worst-case time. For example, if an operator pulls the torch trigger, a burst of air separates the parts. If the operator thereafter releases the trigger (e.g., because an arc was not established or for other reasons) it takes a finite amount of time for the air pressure to dissipate and allow the parts to come back into contact. If the operator pulls the trigger before the parts come together, the system will fail to arc (there was no contact to execute the contact start process). In prior art systems, it may require more than one second for a fifty foot hose to bleed sufficient air to allow the parts to come back into contact. Thus, such systems require a delay of about 1.5 seconds or more to account for this delay. Such a delay is inefficient in torch systems exhibiting a shorter dissipation time (e.g., because of a shorter hose). Similarly, if a particular torch requires more time than the expected worst case time, such prior art systems may prevent that torch from restarting at all because they would allow the torch operator to initiate a start process before the parts return into contact.
It is also known that a failure of the gas control solenoid can prevent the flow of gas. For example, if the gas control solenoid fails to allow gas flow to the torch during the starting process (e.g., because of a component failure of an obstruction in the solenoid), DC voltage may be applied to both the tip and electrode. These elements, however, will not separate if air flow is required to blow the components apart. In some prior art systems, this voltage can exceed xe2x88x9248 VDC (plasma arc torch systems generally operate on a negative voltage basis; the voltage applied at the electrode is negative and the positive power supply output is connected to ground). This can occur in prior art torch systems that employ a pilot resistor (e.g., 1 ohm) between the tip and the positive ground. Further, the tip may be exposed so that the voltage applied to it is likewise exposed.
Another possible problem in blow forward torches can occur during transferred arc operations. If the torch tip is pushed into contact with the workpiece with sufficient force to overcome the gas pressure that normally keeps the tip and electrode separated (i.e., the tip is forced into contact with the electrode) the arc could extinguish but current would continue to flow.
For these reasons, an improved contact start plasma-arc torch system is desired. Such an improved plasma-arc torch system benefits from an improved solenoid control circuit and method that improves the efficiency of the contact start process. Also, a torch system is desired that provides benefits from an improved circuit and method for ensuring that the contact start elements separate as expected upon the presence of flowing gas. Such an improved system allows greater efficiency, for example, by improving reliability and restart capabilities.
The invention meets the above needs and overcomes the deficiencies of the prior art by providing an improved contact start plasma-arc torch system and method. In one aspect, the invention relates to a method of operating a contact-start plasma arc torch system. The torch system includes a torch activation switch indicating desired operational states of the torch, a power supply having first and second output terminals and selectively supplying an output voltage therebetween, a plurality of contact start elements, with a first one of the plurality of contact start elements being in selective electrical communication with the first output terminal, and with a second one of the plurality of contact start elements being in selective electrical communication with the second output terminal. The method includes monitoring the torch activation switch and determining if the desired operational state of the torch transitions from an off state to an operating state. The method also includes causing a condition allowing a closed electrical circuit to be established between the plurality of contact start elements. The method further includes sensing a closed electrical circuit condition between the plurality of contact start elements. The method also includes opening a gas control switch and providing a flowing gas to be ionized upon sensing the closed electrical circuit condition between the plurality of contact start elements. The method further includes sensing an open electrical circuit condition between the plurality of contact start elements after opening the gas control switch, closing the gas control switch and removing the flowing gas if the open electrical circuit condition is sensed between the plurality of contact start elements after the flowing gas has been provided.
In another aspect, the invention relates to a method of operating a contact-start plasma arc torch system. The torch system includes a torch actuation switch indicating desired operational states of the torch, a power supply having first and second output terminals and selectively supplying an output voltage therebetween, a plurality of contact start elements, with a first one of the plurality of contact start elements being in selective electrical communication with the first output terminal, and with a second one of the plurality of contact start elements being in selective electrical communication with the second output terminal. The method includes monitoring the torch activation switch and determining if the desired operational state of the torch transitions from an off state to an operating state. The method further includes closing a gas control switch to allow the plurality of contact start elements to form a closed electrical circuit between said plurality of contact start elements. The method also includes monitoring the output voltage and comparing the output voltage to a low voltage threshold. The method further includes opening the gas control switch and providing a flowing gas to be ionized if the output voltage is less than the low voltage threshold. The method also includes comparing the output voltage to a high voltage threshold after opening the gas control switch and thereafter closing the gas control switch and removing the flowing gas if the output voltage is greater than the high voltage threshold.
In still another aspect, the invention relates to a method of operating a contact-start plasma arc torch system. Such a torch system includes a torch activation switch having a first state and a second state, a power supply responsive to the state of the torch actuation switch and selectively providing an output voltage, and a solenoid selectively allowing a gas to be ionized to flow into the torch. The method includes determining the state of the torch actuation switch; determining a value indicative of the output voltage; determining the position of the solenoid; opening the solenoid if the torch actuation switch is in the second state and the solenoid is closed; and closing the solenoid if the torch actuation switch is in the second state and the solenoid is open and the output voltage is greater than an open circuit threshold.
In yet another aspect, the invention relates to a contact start plasma-arc torch system for use by a torch operator in connection with a workpiece. The torch system includes a power source for selectively providing an output voltage. The output voltage transitions from a first value to a second value when a contact start operation is initiated. A torch head includes an electrode and a tip. The electrode is positioned in a circuit path with the power source and receives the output voltage. The tip is adjacent the electrode. The system also includes a source of gas to be ionized, a gas supply line, and a gas control solenoid associated with the gas supply line. The gas control solenoid is positioned between the source of gas and the torch head and selectively allows gas from the source of gas to flow into the torch head via the gas supply line. A gas monitor circuit provides a gas pressure signal having a parameter indicative of a gas pressure in the gas supply line at a point between the gas control solenoid and the torch head. A control circuit monitors the output voltage and the gas pressure signal. Te control circuit sets a time delay period when the output voltage transitions from the first value to the second value and causes the output voltage to reset from the second value to a third value if after the time delay period the gas pressure signal is less than a gas threshold.
In still another aspect, the invention relates to a contact start plasma-arc torch system for use by a torch operator in connection with a workpiece. The torch system includes a power source for selectively providing an output voltage. The torch system also includes a torch head including an electrode and a tip. The electrode is positioned in a circuit path with the power source and receives the output voltage. The tip is adjacent the electrode. The torch system further includes a source of gas to be ionized, a gas supply line, and a gas control solenoid associated with the gas supply line. The gas control solenoid is positioned between the source of gas and the torch head and selectively allows gas from the source of gas to flow into the torch head via the gas supply line. A gas monitor circuit provides a gas pressure signal having a parameter indicative of a gas pressure in the gas supply line at a point between the gas control solenoid and the torch head. A control circuit monitors a differential voltage selectively established between the electrode and the tip. The differential voltage transitions from a first value to a second value when a contact start operation is initiated. The control circuit also monitors the gas pressure signal. The control circuit sets a time delay period when the differential voltage transitions from the first value to the second value and causes the power source to remove the output voltage from the electrode if after the time delay period the gas pressure signal is less than a gas pressure threshold.
In another aspect, the invention relates to a contact start plasma-arc torch for use by a torch operator in connection with a workpiece. The torch system includes a power source for selectively providing an output voltage that transitions from a first value to a second value when a contact start operation is initiated by the torch operator. A torch head includes an electrode and a tip. The electrode is positioned in a circuit path with the power source and receives the output voltage. The tip is adjacent the electrode. A control circuit monitors a differential voltage between the electrode and the tip. The control circuit causes the output voltage of the power source to transition to a third value if the differential voltage between the electrode and the tip remains less than a contact start threshold after a time period sufficient to allow the contact start operation to complete has elapsed.
In yet another aspect, the invention relates to a power supply suitable for use in connection with a contact start plasma-arc torch system. Such a torch system includes a torch head, a source of gas to be ionized, a gas supply line supplying gas to the torch head, and a gas control switch associated with the gas supply line. The gas control switch is positioned between the source of gas and the torch head and selectively allows gas from the source of gas to flow into the torch head via the gas supply line. A gas pressure switch provides a gas pressure signal indicative of a gas pressure in the gas supply line at a point between the gas control solenoid and the torch head. The power supply includes a power source for selectively providing an output voltage to the torch head. The output voltage transitions from a first value to a second value when a contact start operation is initiated. A control circuit monitors the output voltage and the gas pressure signal. The control circuit sets a time delay period when the output voltage transitions from the first value to the second value and causes the output voltage to reset from the second value to a third value if after the time delay period the gas pressure signal is less than a gas pressure threshold.
In still another aspect, the invention relates to a power supply suitable for use in connection with a contact start plasma-arc torch system. The torch system includes a torch head having an electrode and a tip, a source of gas to be ionized, a gas supply line supplying gas to the torch head, and a gas control switch associated with the gas supply line. The gas control switch is positioned between the source of gas and the torch head and selectively allows gas from the source of gas to flow into the torch head via the gas supply line. A gas pressure switch provides a gas pressure signal indicative of a gas pressure in the gas supply line at a point between the gas control solenoid and the torch head. The power supply includes a power source for selectively providing an output voltage to the electrode. The output voltage transitions from a first value to a second value when a contact start operation is initiated. A control circuit monitors a differential voltage selectively established between the electrode and the tip. The differential voltage transitions from a first value to a second value when the contact start operation is initiated. The control circuit also monitors the gas pressure signal. The control circuit sets a time delay period when the differential voltage transitions from the first value to the second value and causes the power source to remove the output voltage from the electrode if after the time delay period the gas pressure signal is less than a gas pressure threshold.
In another aspect, the invention relates to a power supply suitable for use in connection with a contact start plasma-arc torch system. The torch system includes a torch head having an electrode and a tip, a source of gas to be ionized, a gas supply line supplying gas to the torch head, and a gas control switch associated with the gas supply line. The gas control switch is positioned between the source of gas and the torch head and selectively allows gas from the source of gas to flow into the torch head via the gas supply line. The power supply includes a power source for selectively providing an output voltage to the electrode. The output voltage transitions from a first value to a second value when a contact start operation is initiated. A control circuit monitors a differential voltage between the electrode and the tip. The control circuit causes the output voltage of the power source to transition to a third value if the differential voltage between the electrode and the tip remains less than a contact start threshold after a time period sufficient to allow the contact start operation to complete has elapsed.
Alternatively, the invention may comprise various other methods, circuits, and systems.
Other objects and features will be in part apparent and in part pointed out hereinafter.